1. Field of the Invention
The present invention relates to a printing apparatus used for a printer or a copying machine, and more specifically, an ink jet printing apparatus and an ink jet printing method using an ink jet printing head.
2. Description of the Related Art
Two broad categories of the ink jet printing apparatus discharging ink from a printing head are available according to the type of means of generating energy for discharging ink, provided at the printing head. Specifically, they are one using an electromechanical energy converting element, such as a piezo, and one using an electrothermal energy converting element, such as a heater. Of these different types, an ink jet printing head using thermal energy generated by the electrothermal energy converting element to discharge ink allows ink discharge ports (nozzles) for discharging printing ink droplets (ink droplets) to be highly densely laid out. This design feature makes possible not only printing of high resolution, but also a more compact body and lower cost. Hence, the application in printing apparatuses of various types whether they are for business use or home use.
If the ink jet printing apparatus is left to stand for an extended period of time without discharging ink, however, ink at and around the discharge ports condenses and becomes more viscous due to evaporation of water content of the ink, which makes the ink tend not to be discharged easily. This results in a discharge velocity at the printing head being reduced and a discharge direction becoming disordered. In extreme cases, the printing head may become completely unable to discharge ink.
The ink jet printing apparatus currently put into practical use is therefore provided with a cap that covers the discharge ports in a totally closed condition during times of nonprinting. The cap is also used for performing a suction recovery operation in which the negative pressure slightly sucks the ink to drain out at and around the discharge ports, by applying a negative pressure from a negative pressure source that communicates with the cap into the cap that totally closes the discharge ports immediately before recording etc. Thereby refreshing the ink at and around the discharge ports and maintaining the ink in a condition suitable for discharge.
This suction recovery operation can be executed only while a printing operation is not performed. The ink at and around the discharge ports can, however, condense and become more viscous even while a printing operation is performed. In such a case, the aforementioned problems can occur. It goes without saying that the ink does not condense or become more viscous, as long as it is discharged from each of the discharge ports at all times when the cap is open. The ink is not, however, discharged from all of the discharge ports during the printing operation. Rather, there can be a discharge port or ports, from which ink is not discharged at all even during a period of printing operation depending on the type of image being handled.
As described in the foregoing, the ink may not be discharged properly due to condensation and thickening of ink at and around the discharge ports, resulting in a printing problem, even when a printing command is issued to a nozzle, from which discharge has not been done for an extended period of time. Degradation of image quality arising from a printing problem as described in the foregoing becomes particularly conspicuous, if a portion of an image to be formed is an edge portion or a fine line portion of a figure.
The conventional ink jet printing apparatus is provided with even another arrangement based on the assumption that there will arise a difference in frequency of use among different nozzles during the printing operation as noted in the foregoing. Specifically, referring to FIG. 9, the printing head is moved to a discharge mechanism 5 provided at a position P2 beyond a scanning area involved with the printing operation (ordinary scanning area) E. Each of the nozzles of the printing head is then subjected to a discharge discharge operation of ink not contributing to the printing operation (predischarge), thereby preventing the problem of nondischarge or the like from occurring. Performing the predischarge to recording, such as this one, during the printing operation period, however, leads to an increase in the printing time and ink consumption.
With the trend in the image produced by the recent ink jet printing apparatuses becoming higher and higher definition, the size of each nozzle used in the printing head is becoming more and more miniaturized. With this miniaturized nozzle, the value of (opening area of discharge port)/(volume) increases. As a result, the ink at and around the discharge ports tend to condense at an even more rapid pace due to evaporation of water content of the ink from the discharge ports. The effect of thickening of the ink from condensation greatly affects discharge processes, resulting in an error being produced in a position an ink droplet lands at or nondischarge tending to occur. To keep the ink at the nozzle having a low use frequency in good conditions, therefore, it becomes necessary to frequently move the printing head to the predischarge position. This results in the printing time increasing and the printing speed decreasing.
A detailed study was conducted using ink, or a mixture of dyes, water, and other solvents, packed in a thin nozzle. The fact that a simple thickening of ink occurs from condensation involved with evaporation of water content of the ink from the discharge ports has been confirmed in this study. In addition to this fact, it has also been found that the exposure to air of a surface part of the ink forms a structure, as a result of dyes and other components gathering on a surface layer within a certain range of time period. The structure formed on the surface layer (hereinafter referred to as the ink structure) has a certain dynamic strength, producing a serious adverse effect on flying of ink droplets.
It is nonetheless possible to easily destroy the ink structure formed at and around the discharge ports by increasing the area of the surface portion of the ink at and around the discharge ports. The principle applicable to this phenomenon is based on the principle that is applicable to breaking thin ice on the surface of water by rippling the water. Namely, enlarging the area of the surface portion of the ink causes cracks to occur in (and thus destroys) the ink structure formed at and around the surface of the discharge ports. This, in turn, allows the dynamic strength of the ink structure to be reduced to the extent that the dynamic strength of the ink structure does not affect flying of ink. Once the ink structure is destroyed, it is possible to fly the ink properly for some time from immediately thereafter until a new ink structure is produced. That is, it is possible to extend the time before the discharge problem occurs by simply oscillating the surface of the ink structure so as to increase the area thereof without having to discharging ink.
Means of removing the ink structure formed at and around the surface of the nozzle discharge ports without involving discharge of ink, as that described in the foregoing, are proposed in a number of patent publications as applicable to printing apparatuses employing the piezo printing system mentioned earlier. These publications include Japanese Patent Application Laid-open No. 53-105321(1978), Japanese Patent Application Laid-open No. 55-042809(1980), Japanese Patent Application Laid-open No. 59-164151 (1984), Japanese Patent Application Laid-open No. 03-164258(1991), Japanese Patent Application Laid-open No. 07-178907 (1995), Japanese Patent Application Laid-open No. 09-201960 (1997), and Japanese Patent Application Laid-open No. 09-226116(1997).
In what is called a bubble jet (a registered trademark) method that makes use of the pressure of bubbles formed through boiling of ink, however, a dynamic change can be made in the ink only when bubbles are formed. Further, once bubbles are formed, the pressure involved in forming the bubbles causes the ink to be invariably discharged.
For this reason, with the conventional ink jet printing apparatus using what is called the bubble jet method, it has been considered to be extremely difficult and impractical to destroy the ink structure formed at and around the surface of the discharge ports by oscillating the ink to the extent only that the oscillation does not allow the ink to be discharged.